1. Field of the Invention
The present invention relates to post-tensioning concrete anchor assemblies and, more particularly, to an anti-corrosive anchor plate assembly adapted for both terminal end and intermediate anchor utilization.
2History of the Prior Art
The prior art is replete with anchor plate assemblies adapted for the securement of post-tensioning tendons thereto for the tensioning of concrete structures. Engineering in the post-tensioning of concrete is a well developed technology and the utilization of such tendons extending through a concrete slab or beam is conventional. The tendons provide structural strength for the concrete in a manner and at a cost not heretofore possible with conventional rebar construction. Utilization of such tendons does, however, require anchor assemblies on opposite ends thereof. The anchor assemblies secure the ends of the tendons extending through the concrete bed and must remain effective during the lifespan of the construction. The effectiveness requires the protection of the tendons which are usually made of steel or the like, from corrosion. Corrosive forces are well known to cause deterioration in the strength of the concrete if allowed to jeopardize the integrity of the tensioning member. To prevent corrosion of the tendon, the steel fibers are usually sheathed in a plastic membrane throughout the length of the slab. The membranes do, however, require termination at the point where the tendons are secured within the anchor assemblies. The reason is obviously to provide appropriate structural integrity at the secured position.
In the process of post-tensioning, it is important that the tendon is free to move within the hardened concrete so that the tensile load on the tendon is evenly distributed along the entire length of the structure. Methods used to assure that the tendons provide free movement within hardened concrete include laying a number of strands of wire in a duct or tube. It is within this duct or tube that the strands of wire are stressed after the concrete is hardened. The duct or tube may be formed of metal or plastic and is usually filled with grease. The prior art also includes parallel strands of wire covered with grease and then covered with spirally wound paper. In some cases the wound paper is replaced by wound or wrapped plastic. Just the opposite is true of prestressing concrete cables. The cables are generally exposed to the concrete and are not covered with the sheath due to the fact that it is important that the concrete bond directly to the metal cable in its prestressed condition. Once the forms are set, the tension in the cable is also established so that once the concrete cures it forms a bond directly to the cable and no cable movement is allowed without movement on the concrete itself. There are, of course, advantages to both systems depending on the type of fabrication utilized in the ultimate application.
The present invention pertains to the anchor assemblies utilized in post tensioning configurations. Prestressing configurations do not require such anchor assemblies because the cables are cemented in the concrete and can simply be cut off. Post tensioning, however, incorporates the duct or plastic tube as described above and the utilization of anchors from opposite ends to produce the tensile force transmitted therethrough. Several prior art patents address such anchor assemblies for the post tensioning of a tendon in a concrete structural component. U.S. Pat. No. 4,363,462 issued to Wlodkowski et al. on Dec. 14, 1982 teaches one such anchor assembly. This particular structure also incorporates a recoverable part having an axially elongated sheath which closely encloses a tendon over a portion on the length of the sheath. When assembled in the formwork, one end of the sheath is arranged to be located within the concrete when it is poured and the other end is located on the exterior of the formwork. By removing the fastening means after the concrete has been poured, it is possible to remove the form and a cup member formed integrally with the sheath to form at least a portioned recess in the concrete member. Tensioning is then provided by conventional means with the tendons secured by wedges or the like in an orifice formed in the anchor plate. As set forth in this reference, it is an important consideration that the tendons be sufficiently protected against corrosion and this is affected by enclosing them in a plastic coating. It is likewise important to cover all areas of the tendon to provide protection against corrosion anywhere therealong. The most sensitive area of corrosion is in the vicinity of the locking wedges in the anchor plate and, therefore, that too is an area for which a sealant or covering must be applied. It may thus be seen that great care has been afforded the prior art structures by providing threaded and similar sealed engagement between the sheath coverings and the anchor assemblies.
There are numerous other approaches to post tensioning anchors for prestressed concrete as set forth and shown in the prior art. U.S. Pat. No. 4,121,325 to Bruinette et al. is a 1978 reference which teaches an anchor and coupling unit for use with stressing cables and reinforced concrete structures. In this particular embodiment the cable anchoring equipment addresses both pretensioned or post-tensioned, prestressed cementuous structures. As stated above, protective sheaths are located around the cables so that the latter may elongate under tension within the concrete. One end of the cable is anchored to the structure and this end is called the dead end. The other end of the cable called the "live end" includes a cable anchor on which the tensioning force may be applied. As discussed, it is necessary to protect both the dead and live anchoring ends of the cables because at these points the sheathing must be peeled back to enable the looking wedges or jaws of the tensioning mechanism to directly engage the cable. When the sheathing is stripped back it is necessary then to protect the cable because the strain induced upon the cable by the clamping jaws and/or wedges is the area in which corrosion will first manifest itself. Failure at this area means failure in the tensioning cable. Of primary consideration, however, is not simply the anchor assembly that is used on opposite ends of a concrete slab, beam or similar poured structure. It is well known to seal, grout or likewise cap the ends of the post-tensioning cables. Problems often arise when a series of contiguous slabs or structural members are poured seriatim with a continuous cable extending therethrough. Due to the length of the member, sections must be poured in discrete quantities and then individually post-tensioned. Once applied, the second contiguous section, must likewise be post-tensioned after pouring. This requires the utilization of an anchor or coupling assembly which allows both the post-tensioning of the first slab as well as the continuation of the tendon through the second slab or beam and the post-tensioning thereof. Such assemblies must also facilitate constructional mandates relative to the ease of use, cost and number of parts available for various jobs. A single anchor assembly which is adapted for both external and intermediate use and which is constructed for protecting the sheath cable for post-tensioning in the manner described above is a very necessary step.
Finally, U.S. Pat. No. 4,82,474 sets forth and describes several improvements in the post tensioning industry. Means are shown therein for fixedly securing the tendon and protecting it with a tubular extension member. The use of an anchor plate specifically constructed for such tubular members and protective caps was a marked advance in the industry. Several problems yet remain in the prior art. The position of the cap over the post tension anchor is often not secure. It would be an advantage to have a cap which could be easily secured to the anchor in the "capped" position. Another problem is the fact that in order to prevent corrosion, the cap is often filled with grease. This takes both time, labor and an excessive amount of grease, which is generally wasted. If a cap were not hollow, such a waste of grease would not occur. The hollow cap, however, is necessary to fit over the tendon extending from the anchor, which overcomes a problem existing in certain prior art designs. A similar problem exists for the tubes themselves. The tubes that cover the cable must also be filled with grease. Again, the grease is expensive and the pressure of the grease expanding into a large empty volume takes time, labor and can often result in misapplication. The securement of the cap and other members during the grease filling operation of the tube requires that the cap and tube be secured to the anchor plate. It is often a problem to maintain the assembly in prior art configurations. In particular, it is necessary that the cap and tube remain secured to the anchor plate and that the joints remain sealed in the anchored configuration after filling. Grease is utilized because it has inherent corrosion inhibitors, but this approach is fraught with problems.
The present invention provides an advance over the prior art by providing a post-tensioning anchor assembly that effectively inhibits corrosion of the post-tensioning anchorage system. The connector tube of the present invention is adapted for receiving expanding foam after engagement with the anchor body. This foam replaces the more expensive grease used in the prior art. Corrosion inhibitor is mixed with the foam as well as applied to the cable prior to injection of the foam. The invention further provides a method for injecting the foam into the connector tube such that it completely fills the void within the tube. The use of inert foam realizes benefits not seen with the grease used in the prior art in that the costs and waste are minimized. A water blocking ring appears on the exterior surface of the tube to minimize the travel of water from the anchor assembly to the small end of the tube. The small end of the connector tube includes a lip which meets with the sheathed cable to create a water tight seal further preventing the chance of introducing corrosive elements to the anchor assembly joint. While the prior art employs terminal end caps, the present invention utilizes an advanced snap-on cap means in conjunction with a segmented foam insert to insure a water tight seal. By incorporating these corrosion inhibitors, the present post-tensioning system advances the art by affording increased reliability, economy and life-span.